Process of forming an electrophotographic element

ABSTRACT

A PROCESS, AND THE PRODUCTS THEREOF, FOR IMPROVING THE DISPERSION OF A PIGMENT INRESIN BINDERS IN THE MANUFACTURE OF ELCTROPHOTOCONDUCTIVE RECORDING ELEMENTS USED IN ELECTROPHOTOGRAPHY BY REGULATING THE TEMPERATURE AND THE ADDITION OF INGREDIENTS DURING SUCH DISPERSION.

June 20, 1972 cs. 0. CASAVANT I 3,671,234

PROCESS OF FORMING AN ELECTROPHOTOGRAPHIC ELEMENT Original Filed Nov. 2, 1967 i 2 Sheets-Sheet 1 GMS-264 (HAS-269 cns-27o ASB-5I6 g |oo- I0 I l l I I l l l l I 2 3 4 10 20 so 4bso k1o ksql mom RESIN GEORGE D. CASAVANT A T TOR XE Y June 20, 1972 G.'D. CASAVANT 3,671,234

PROCESS OF FORMING AN ELECTROPHOTOGRAPHIC ELEMENT Original Filed Nov. 2, 1967 2 Sheets-Sheet 2 %BY WEIGHT TOTAL RESIN VS. VISCOSITY IN GPS (BROOKFIELD) My 1W L 1 SWSITY ens-27o VISCOSITY TOTAL m \HSCOSITY cus-zeevvscosm 3 2 2 g 4 T. 2 g 5 2800 7600 6 g 53 8 e s 2 I 1 20 3530 as 400 V vs FIG. 2

GEORGE D. GASAVANT ATTORNEY United States Patent 3,671,234 PROCESS OF FORMING AN ELECTROPHOTO- GRAPHIC ELEMENT George D. Casavant, Gansevoort, N.Y., assignor to International Paper Company, New York, N. Continuation of abandoned application Ser. No. 684,587, Nov. 2, 1967. This application Feb. 8, 1971, Ser. No.

Int. Cl. G03g 5/08 U.S. Cl. 961.8 2 Claims ABSTRACT OF THE DISCLOSURE A process, and the products thereof, for improving the dispersion of a pigment in resin binders in the manufacture of electrophotoconductive recording elements used in electrophotography by regulating the temperature and the addition of ingredients during such dispersion.

CROSS-REFERENCE TO EARLIER APPLICATIONS Portions of the material discussed herein are covered by co-pending U.S. Ser. No. 586,938, filed Oct. 17, 1966, now abandoned.

This application is a continuation of application Ser. No. 684,587, filed Nov. 2, 1967, and now abandoned.

I BRIEF SUMMARY OF THE INVENTION This invention relates to the field of electrophotography and, more particularly, to a process of improving the quality of coating compositions used on electrophotoconductive recording elements and the products of such process.

The principles of electrophotographic reproduction have been well developed in the prior art. They generally comprise placing a uniform static charge on a recording element by one of several methods and subjecting the element to light from an imaging system or a contact system to dissipate the charge selectively, thus producing an electrostatic image, which image is subsequently converted into a visible permanent record by the application of toner.

The present invention is concerned with an electrophotoconductive recording element consisting of a multilayer sheet. The first layer is composed of a base sheet of cellulosic material, e.g., paper, which may or may not contain material to improve the conductivity of the base sheet. Over the base paper is coated a second layer containing a material of high electrical conductivity. The third layer consists of a resinous binder of high electrical resistivity in which is dispersed a pigment such as zinc oxide.

The utilization of this type of electrophotoconductive recording element or media has been commercially perfected to the point of finding good acceptance in the field of reproduction. This method of reproduction involves the following steps:

(1) Charging the electrophotoconductive sheet by showering the recording media with negative ions from a charging device such as a corona;

(2) Selectively discharging the electrophotoconductive sheet by exposing it to light projected from or transmitted through the original document which is being reproduced (those areas with the most exposure to light become the most discharged);

(3) Toning of the latent image with a toner which consists of a mixture of resinous materials and colored dye particles in a finely divided state suspended in air, in a liquid carrier, or supported by metal filings (depending Patented June 20, 1972 on the polarity of the particles and the charge on the electrophotoconductive sheet, the particles will adhere differentially to the surface of the sheet producing gradations of gray); and,

(4) Fusing of the image to the recording media eithel thermally or chemically with solvent vapors.

Electrophotoconductive recording elements which can be used in the process just reviewed have been disclosed in many U.S., as well as foreign, patents. In most cases, they include a resinous binder which is made up of one or more than one of the following materials: polysiloxanes, cellulose esters, polyvinyl acetate, acrylics, polystyrene, polyesters, to name a few. It is generally accepted in the art that a single resin binder does not, in most cases, have all the physical, chemical or electrical properties that are necessary to produce a good recording media. In many cases, a combination of two or more compatible resinous materials will produce a more desirable electrophotographic binder than a single resin.

It was recently found that a greater degree of electrophotographic efliciency can be realized when the two resinous materials are incompatible. See copending U.S. Ser. No. 586,938, filed Oct. 17, 1966. The compatibility of a resin combination can be determined by mixing the resins together and noting the nature of the mixture. A clear homogenous mixture indicates a compatible system while, on the other hand, an incompatible system will have an appearance ranging from cloudy solution to an insolublized or precipitated resin mix.

It was recently found that a drastic incompatibility situation may introduce problems in zinc oxide dispersion. However, by using the proper solvents, the incompatibility can be sufficiently buffered to allow normal compounding of the resins and zinc oxide. Incompatibility may be in the nature of a mutual rejection of each of the resinous species, the robbing of one resins solvent by the other, or an intermolecular reaction which results in the formation of an insoluble molecular species. It has been found that low molecular weight ketones are very eflicient in acting as co-solvents from the incompatible resins. The incompatibility of the quasi-solvated resin can be proven by allowing a thin deposition of the resin mixture to dry on glass. The foggy appearance of the film confirms its incompatible nature.

The amount of dispersion of the zinc oxide has no direct relationship to the resins incompatibility. The dispersion of the zinc oxide is goverened more by the smoothness requirements than by electrical or sensitometric demands. The amount of grind necessary for a pleasing sheet is subjectively arrived at by noting the type of coating produced by hand drawndown during the course of disper sion. A less subjective method of evaluating the grind time is to evaluate the coating at various time intervals with a fineness of grind gage. The Precision Gage and Tool Company of Dayton, Ohio produces this type of gage. Generally a grind of 5 mil or less is considered acceptable for electrophotographic work.

However, the degree of incompatibility does enter into the power required to disperse the zinc oxide, so that the more incompatible the resins, the more power or energy is needed to get a desired level of dispersion. And, it has now been found that increasing the input of mechanical energy to achieve better dispersion of the zinc oxide can trigger an undesirable side reaction wherein diluents, usually aromatics like toluene or xylene, which are added for economic reasons, are rejected by the resin-Zinc oxide combination. The result is a gummy mass, rather than a slurry in which the finely particulate Zinc oxide is uniformly dispersed.

The just-described rejection phenomenon is especially acute when polyvinyl acetate (PVAc) and its related copolymers and multipolymers are used in the production of resin binders for electrophotoconductive recording elements. The product formed when a commercially available PVAc-type resin (wherein ethanol or ethyl acetate is the true solvent) and another resin are mixed is not easily bulfered by ketones or other solvents. At the outset, the PVAc family of resins has a much lower tolerance to diluents conventionally used in electrophotographic coatings and, as indicated, the introduction of mechanical energy readily overwhelms this tolerance and upsets the delicate balance or equilibrium of the resin system.

Just why the PVAc family of resins, and probably others, are so intolerant of diluents or so unstable under the circumstances described is not certain. However, it has been theorized by resin manufacturers that the moisture content level of the resins is the cause of these problems. They suggest that a moisture content level in a certain range gives rise to these problems, while a moisture content level above or below such range minimizes these problems.

It is the object of the present invention to overcome or obviate the tendency of certain resins useful in the production of electrophotographic coatings to be intolerant of diluents therefor and to be unstable when it is sought to disperse a pigment such as zinc oxide therein with the aid of mechanical energy.

Broadly, the invention resides in the discovery that the dispersion of the zinc oxide in a resin binder, particularly when it is comprised of two or more incompatible resins, is improved when it is conducted at a reduced temperature and when it is conducted in discrete stages, the first of which comprises initially dispersing the zinc oxide pigment in a small fraction of the total resin binder employed in the electrophotographic coating.

Conventionally, the dispersion of zinc oxide in a resin binder is accomplished by the use of high-speed impeller which grinds the mixture, adds energy to it, and raises its temperature to about 140 F. In accordance with the present invention, a heat exchanger or the like is used to keep the temperature of the mixture substantially lower.

More particularly, the preferred temperature at which the dispersion of \zinc oxide is conducted in accordance with the present invention is in the range of about 55 F. to just above the dew point at room temperature if such dew point is higher than 55 F. This temperature range is significant, because it is believed to alter least the moisture content of the resins involved and, thereby, to jeopardize least the delicate equilibrium between the resins, their moisture content, and the diluents used therewith in the preparation of electrophotographic coatings. For similar reasons, the dispersion of the zinc oxide in a small fraction of the total resin binder is also significant.

The addition, with mechanical mixing, of a resin (e.g., PVAc) to a given quantity of zinc oxide pigment in a diluent causes the viscosity of the resulting mixture to decrease sharply and the degree of zinc oxide dispersion in the mixture to increase sharply, and very possibly reach a maximum level. Further addition of resin, with continued mixing, causes the viscosity of the mixture to increase and the degree of zinc oxide dispersion therein, which is at least roughly inversely proportional to such viscosity, decreases. This relationship is demonstrated in attached Table I and, on the basis thereof, graphically portrayed in the attached figure wherein the viscosities measured in centipoises (Brookfield) of four resins AVinac ASB516, an alkaline soluble polyvinyl acetate made by Air Reduction and Carbide Company; BGelva Multipolymer Solution (GMS) 264, an ethanol solution of a polyvinyl acetate-maleate copolymer made by Monsanto Chemical Company;

C-Gelva Multipolymer Solution (GMS) 269, a polyvinyl acetate-maleate copolymers in a blend of isopropyl acetate and toluene made by Monsanto Chemical Company; and,

D-Gclva Multipolymer Solution (GMS) 270, an ethanol, solution of a self-curing polyvinyl acetate copolymer are shown to drop at first when they are added to a fixed quantity of zinc oxide pigment and then when from about 5% to 20% by weight of the total resin needed to achieve the ultimately desired 6 parts by weight of pigment to 1 part by Weight of resin have been added, the viscosities are shown to rise. This sort of relationship has also been found to apply to other resins, e.g., alkyds (Archer/ Daniels/Midland 3204 and De Soto B70-028) and acrylates (Archer/Daniels/ Midland 3205, Rohm & Haas AT- 50 and B72, and B. F. Goodrich Carboset 514-A). And the consequence or meaning of this relationship in the present frame of reference is that, after a certain small quantity of resin has been added, the etficiency of mechanical mixing diminishes (in terms of zinc oxide dispersion) and further mechanical mixing introduces excess energy which, presumably in the form of heat, can disturb the tender equilibrium of resin, moisture contact, and diluents. Hence, only a limited measure of mechanical mixing is in order and, by deliberately mixing the zinc oxide pigment with a small fraction of the total resin binder in accordance with the present invention, one can achieve a maximum of dispersion of the zinc oxide in the resin with a minimum of mechanical mixing and can diminish the risks of disturbing the balance of the system. And one can then proceed with the addition of the remaining resin needed, knowing that mechanical mixing can and should be sharply controlled lest it prove deleterious.

Thus, it has been found that, where 6 parts by weight of zinc oxide and 1 part by weight of GMS 270 polyvinyl acetate copolymer resin are to be dispersed in toluenexylene, a fraction of the resin can be used to disperse the zinc oxide and is as etficient in this role as a commercially available dispersant such as sorbitan monooleate. For the GMS 270 resin, the optimum resin concentration was found to be from about 7% to 15% by weight of the total resin to be added. This quantity is equivalent to from about 1.25% to 2.5% by weight, based on the zinc oxide used.

Dispersions conducted using the lower temperature and predispersion with part of the resin described above have resulted in greatly improved stability, a reduction of as much as 30% of grind time andsmoother grind. This is true with respect to the following coating dispersion example, the coated paper product of which is competitive with and can be substituted for commercially available sheets designed for the A. B. Dick electrophotographic copier.

EXAMPLE A photoconductive mixture of incompatible resins is prepared by the following procedure wherein quantities are given in parts by weight:

50 parts of photoconductive zinc oxide (American Zinc Sales Companys AZOZZZ66133), 1.25 parts of GMS 270 polyvinyl acetate copolymer, 30.00 parts of toluene, and 10.0 parts of xylene are introduced into a Kady mill. The mixture is mechanically dispersed therein for from 5 to 10 minutes with an external heat exchanger keeping the mixture at a temperature below about 60 F.

After the coating was dispersed for the indicated time, a sample was withdrawn and evaluated for smoothness with a fineness of grind gage and by drawing down the coating on aluminum foil for examination under a magnifying glass. Once the quality of dispersion had been ascertained, the following ingredients were added under agitation: 6.67 parts of GMS 270, 0.41 parts of a styrenated alkyd (i.e., De Soto Chemical Coating, Inc.s De Soto E-7013A) at 55% total solids (T.S.), and 1.2 parts of methyl ethyl ketone. The resulting formulationhad a viscosity reading of 76 centipoises on the Brookfield scale immediately after dispersion and was coated on paper by conventional means.

What is claimed is:

1. In the manufacture of an electrophotoconductive element having a layer in which photoconductive zinc oxide is dispersed in a resinous binder,

said resinous binder including a blend of a first resin and a second resin which is soluble in the same solvent as said first resin,

said first resin being selected from the group consisting of polyvinyl acetate, polyvinyl acetate maleate, and self curing polyvinyl acetate copolymers and, when zinc oxide is increasingly dispersed therein, forming a dispersion having a viscosity which initially diminishes to a minimum viscosity and then increases, said second resin being electrically insulating and at least partially incompatible with said first resin,

said incompatibility being demonstrable by drying a thin deposition of said blend on a piece of test glass until it has a foggy appearance which confirms said incompati- 20 bility, the improvement comprising the step of initially dispersing the photoconductive zinc oxide in a fraction of said first resin while maintaining the resulting dispersion at a temperature of about 55 F. to just above the dew point if such dew point is higher than 55 F.

2. The method of claim 1 wherein said fraction of said first resin is in the range of about 1.25% to 2.5% by weight of said photoconductive zinc oxide.

References Cited CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R.

117-34, 161 UC; 252--50l 

